Research XXL: nature as laboratory

Nature determines when things get underway

What happens inside a debris flow? When does a stream transport rock and debris? Large-scale experiments in mountain streams provide answers.

The noise is deafening. A passing mountain biker stops and looks around in bewilderment. A sign under the flashing warning light warns: “Danger! Debris flows can occur at any time – even when the weather is good.” Only when WSL technician François Dufour calls out: “Don’t worry, it’s only a test”, does the biker continue riding along the almost completely dried up stream bed. No one thinks that the trickle between gravel and debris, in Illgraben near Leuk (Valais), will cause any harm. However, it can turn into a deadly wave of mud and water in a matter of seconds. When it rains, sometimes combined with snowmelt, hundreds of tons of mud, gravel and boulders thunder down into the valley as a ­debris flow. This sort of debris flow is more powerful than a flood with sediment and can cause a correspondingly high amount of damage. Thanks to the geological structure of Illgraben, several of these events ­occur each year, which makes it an ideal place to study debris flows.

Since May 2000, WSL has had a number of measurement instruments installed in Illgraben, including a geophone that measures seismic ground motion. The geophone records the ­vibrations that signal the beginning of a debris flow and automatically starts video recording. Radar and ­lasers record the flow height and speed of the mud and debris mass in several locations. At the outlet of the torrent channel, where the Illgraben flows into the Rhone, sensors in a side wall measure the forces inside the debris flow mixture. And underneath the bridge that carries the cantonal road, a scale records the weight of the ­material flowing past.

“This location provides us with a unique opportunity to research the ­inner workings of debris flows: their composition, their flow behavior, their characteristics,” explains technician Dufour. The data allows researchers to improve their understanding of the processes that occur during a debris flow and optimize computer simulations. This helps engineers and planners ­prepare hazard maps and assess any protective measures.

WSL also measures how much material may be carried in a stream in Alptal (Schwyz), albeit for a different reason. Here, it rains often and heavily, and as the soil is clay-rich, the water does not drain well. So the risk of flooding is high. Although this may pose a risk to the surrounding communities, it is a boon to researchers: The area has the ideal conditions in which to explore the factors that contribute to flooding. In the 1960s, WSL equipped various mountain torrents in the Alptal with measurement devices to find out how the forest affects the formation of floods and water quality. Researchers have also been looking into how sediment and driftwood are transported, among other issues, since the 1980s and have developed models for forecasting floods for several years.

Dieter Rickenmann, a researcher in the mountain torrents group, stands with two Japanese colleagues looking at the Erlenbach stream in Alptal. ­Despite the sun, it is cool, the air is damp and their fingers quickly become clammy. When they reach the large retention basin, Rickenmann shows his guests how WSL researches sediment transport in one of the most active mountain torrents in Switzerland. There is a reason the visitors have traveled so far: The WSL’s sediment measurement equipment is not found anywhere else in the world.

WSL workers developed a method to monitor and measure the sediment ­indirectly. The geophone records the vibrations that sediment particles cause when they roll over steel plates. The steel plates are embedded in the streambed, with the geophone sensors mounted on their underside. The signals are calibrated using direct measurements of the transported sediment. To do this, the re­searchers attached collecting baskets at the inlet of the retention basin. These baskets are mounted on horizontal rails and are pulled into the flow of water automatically by winches whenever the flow is strong. There, they collect sand, gravel, and cobbles transported along the stream bed. The ma­terial ­collected in the ­retention basin is also used to calibrate the measurements of the ­geophone, by measuring the deposited sediment.

“Using these measurements, we were able improve the calculation methods for sediment transport significantly and develop a computer simulation model. We can now use this model to calculate how much sediment will be transported under what conditions – even for longer stretches of mountain streams,” explains Rickenmann. The model was used as part of National ­Research Program 61 ‘Sustainable Water Management’. Researchers looked at how climate change might affect sediment transport and the living con­ditions for the brown trout.

The collecting baskets remain motionless today, as there is hardly any water in the Erlenbach, let alone sediment transport. The fact that the mountain stream can look quite different is documented by the photos from the last major flood in June 2007. Following a rainstorm, the Erlenbach filled the inlet basin with stones, mud and driftwood in about two hours. “This sort of extreme event takes place here about every 10 years. We have about 15 to 20 floods with sediment transport annually,” says Rickenmann. Fortunately, the stream did not cause any damage to the nearby hamlet of Brunni.